[Gameplay] Kalista Rend Not Resetting Properly

I was playing Kalista after she was re-enabled and noticed that I was having a hard time using her rend to last hit. I frequently would try to use it only to see it on cooldown, despite my last use of it being used to kill a minion. I thought My hand was slipping, or I had misread the tooltip (the "up to twice" part is a little confusing) But it seemed that Kalista's rend was going on cooldown randomly after a reset, as though it was being used again automatically.

Steps to reproduce: Play Kalista, Use rend to kill minions, watch for it to suddenly go on cooldown.

I was playing on a 2009 Macbook Pro (Intel core 2 duo, Nvidia GeForce 9600M, 4GB Ram, Mac OS 10.9.5) though I doubt it's relevant.